How does a query tree represent a relational algebra expression?     

Ans: This includes transforming an initial expression (tree) into an equivalent expression (tree) that is more proficient to execute. Two relational algebra expressions are said to be equal if the two expressions produce two relation of the same set of attributes and consist of the same set of tuples even though their attributes may be ordered in a different way.

 The query tree is a data structure which denotes the relational algebra expression in the query optimization process. In the query tree the leaf nodes correspond to the input relations of the query. The internal nodes denote the operators in the query. While executing the query, the system will execute an internal node operation when its operands available after that the internal node is replaced by the relation which is acquired from the preceding execution.